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88 lines
3.6 KiB
ReStructuredText
88 lines
3.6 KiB
ReStructuredText
Flows
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=====
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.. topic:: Summary
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* *Flows automate the process of agreeing ledger updates*
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* *Communication between nodes only occurs in the context of these flows, and is point-to-point*
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* *Built-in flows are provided to automate common tasks*
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Video
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-----
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.. raw:: html
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<iframe src="https://player.vimeo.com/video/214046145" width="640" height="360" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>
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<p></p>
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Motivation
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----------
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Corda networks use point-to-point messaging instead of a global broadcast. This means that coordinating a ledger update
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requires network participants to specify exactly what information needs to be sent, to which counterparties, and in
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what order.
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Here is a visualisation of the process of agreeing a simple ledger update between Alice and Bob:
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.. image:: resources/flow.gif
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:scale: 25%
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:align: center
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The flow framework
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------------------
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Rather than having to specify these steps manually, Corda automates the process using *flows*. A flow is a sequence
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of steps that tells a node how to achieve a specific ledger update, such as issuing an asset or settling a trade.
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Here is the sequence of flow steps involved in the simple ledger update above:
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.. image:: resources/flow-sequence.png
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:scale: 25%
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:align: center
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Running flows
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-------------
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Once a given business process has been encapsulated in a flow and installed on the node as part of a CorDapp, the node's
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owner can instruct the node to kick off this business process at any time using an RPC call. The flow abstracts all
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the networking, I/O and concurrency issues away from the node owner.
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All activity on the node occurs in the context of these flows. Unlike contracts, flows do not execute in a sandbox,
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meaning that nodes can perform actions such as networking, I/O and use sources of randomness within the execution of a
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flow.
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Inter-node communication
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^^^^^^^^^^^^^^^^^^^^^^^^
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Nodes communicate by passing messages between flows. Each node has zero or more flow classes that are registered to
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respond to messages from a single other flow.
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Suppose Alice is a node on the network and wishes to agree a ledger update with Bob, another network node. To
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communicate with Bob, Alice must:
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* Start a flow that Bob is registered to respond to
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* Send Bob a message within the context of that flow
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* Bob will start its registered counterparty flow
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Now that a connection is established, Alice and Bob can communicate to agree a ledger update by passing a series of
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messages back and forth, as prescribed by the flow steps.
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Subflows
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^^^^^^^^
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Flows can be composed by starting a flow as a subprocess in the context of another flow. The flow that is started as
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a subprocess is known as a *subflow*. The parent flow will wait until the subflow returns.
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The flow library
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~~~~~~~~~~~~~~~~
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Corda provides a library of flows to handle common tasks, meaning that developers do not have to redefine the
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logic behind common processes such as:
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* Notarising and recording a transaction
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* Gathering signatures from counterparty nodes
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* Verifying a chain of transactions
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Further information on the available built-in flows can be found in :doc:`flow-library`.
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Concurrency
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-----------
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The flow framework allows nodes to have many flows active at once. These flows may last days, across node restarts and even upgrades.
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This is achieved by serializing flows to disk whenever they enter a blocking state (e.g. when they're waiting on I/O
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or a networking call). Instead of waiting for the flow to become unblocked, the node immediately starts work on any
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other scheduled flows, only returning to the original flow at a later date.
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